Cole Crops, Root, Bulb and Leafy Vegetable Edition by
Jennifer Allen, Vegetable Crop Specialist, OMAFRA, Guelph

Downy mildew of onions is a water mould that does well under cool to moderate temperatures <22°C), with prolonged leaf wetness periods.  Early identification is difficult because symptoms often don’t’ appear until 5 to 7 days after infection.  Infected plants are often found in patches, but the disease can quickly spread through the entire field.  Diseased leaves turn pale-green/yellow and then collapse and die.  

Although downy mildew is often considered a sporadic disease, it can explode and be very destructive to onions.  Infection occurs when onion foliage remains wet for 2–6 hours at 3°C–14°C (37°F–57°F). The incubation period lasts for 10–16 days.  A relative humidity of 95% or greater promotes further infections.  Research has shown that downy mildew spores germinate at night and spread during the day by wind and/or rain. 

While a forecasting model called DOWNCAST has been developed to help predict downy mildew outbreaks it is not available in all onion growing regions.  So, if the forecasting model is not available, it is recommended that a protective fungicide program be initiated during cool and humid weather conditions.  Like those we are currently experiencing.

Early downy mildew symptoms	Typical disease symptoms	Outcome of an infection

Here are the degree and growing degree-day accumulations for April 1 – July 24, 2009. Yellow highlights areas where 2nd generation accumulations have been reached.

Area	Onion Maggot	Cabbage Maggot	Carrot Rust Fly	Carrot Weevil	Tarnished Plant Bug	Leaf-hopper	GDD
Bradford/ Holland Marsh*	1020.0	n/a	1125.6	712.2	300.1	532.1	915
Sarnia	1067.5	863.8	1173.7	767.5	359.0	588.0	964
London	1136.8	926.5	1245.3	825.7	389.1	637.7	1008
Waterloo	1008.5	805.4	1114.8	710.0	301.3	534.7	907
Windsor	1392.8	1175.0	1504.7	1070.0	587.0	868.9	1282

Degree-day information for pests. 

	Degree Days
	1st Generation	2nd Generation	3rd Generation
Onion Maggot	210	1025	1772
Cabbage Maggot	314-398	847-960	1446-1604
Carrot Rust Fly	329 – 395	1399-1711	n/a
Carrot Weevil	138 – 156 455 (90% oviposition)	No model available	n/a
Tarnished Plant Bug	40	n/a	n/a
Aster Leafhopper	128	390	n/a

DISEASE FORECASTS  

BOTCAST —  Disease severity index  is 28.  Risk of developing Botrytis is moderate to high.     

DOWNCAST—Predicted sporulation infection period for the last three days.  Risk of downy mildew on onions is high.  Downy mildew has been found in onions in the Holland Marsh.

BREMCAST—Predicted sporulation infection period in the last three days.  Risk of downy mildew on lettuce is moderate to high.  

Data provided thanks to funding from the Holland Marsh Growers’ Association, Bradford Co-op Storage Ltd., Muck Crops Research Station, Department of Plant Agriculture, University of Guelph., Lake Simcoe Conservation Authority, Bayer CropScience, BASF, DuPont, Engage Agro and Syngenta.